"Our study found that women who were using contraception containing estrogen tended to have higher vitamin D levels than other women," said Harmon in a statement released by the Endocrine Society.

Harmon also noted that the study findings revealed that women who stopped taking birth control may be at an increased risk for vitamin D deficiency when they decide they want to become pregnant.

"For women who are planning to stop using birth control, it is worth taking steps to ensure that vitamin D levels are adequate while trying to conceive and during pregnancy," Harmon added.

"These data indicate that there is an interaction between 'normal' estrogen, 'high' estrogen levels, and vitamin D levels," commented Nanette Santoro, MD, professor and E Stewart Taylor Chair of Ob/Gyn at the University of Colorado School of Medicine, who was not involved in the study.

"The overall lower Vitamin D in women who go off of birth control pills to conceive in this study is quite low and may indicate that more supplementation should be considered when women are trying to get pregnant."

Harmon and colleagues investigated the relationship between the use of exogenous hormones and vitamin D status using data from the Study of Environment, Lifestyle & Fibroids (SELF), a cohort of African-American women, ages 23-24, who lived in Detroit, MI, or the surrounding area.

At enrollment all 1,662 participants provided blood samples, which were analyzed to measure levels of 25-hydroxy vitamin D (25(OH)D), the primary circulating form of vitamin D. During this clinical visit participants also completed three questionnaires -- a 24-hour questionnaire, 4-week questionnaire, and Block 2005 Food Frequency Questionnaire -- regarding their use of prescription and over-the-counter medications, vitamin D supplements, contraceptives, as well as dietary intake of vitamin D.

Additionally, researchers relied on telephone and computer-based questionnaires for data on covariates such as the amount of time participants spent outdoors and vacations in sunny locations during the winter.

Harmon and colleagues found that overall, serum 25(OH)D concentrations were low, median 15.7 ng/mL, which is well below the Institute of Medicine's recommendation of 20 ng/mL.

They also concluded that use of a supplement containing vitamin D was common, 40%, yet it varied with factors such as education, smoking, and time spent outdoors.

After adjustment, researchers found that the current use of estrogen-containing contraceptives was associated with a 20% higher 25(OH)D level (95% CI 14 to 27).

There was no increase in vitamin D levels among those who'd used estrogen in the past, but were not current users, suggesting the results were unlikely to be due to unmeasured confounding by factors related to contraceptive choice, Harmon and colleagues reported.

"We could not find any behavioral differences such as increased time spent outdoors to explain the increase," the researchers wrote. "Our findings suggest that contraceptives containing estrogen tend to boost vitamin D levels, and those levels are likely to fall when women cease using contraception."

For Santoro, one of the biggest takeaways from the study is the importance of thinking about vitamin D levels when trying to conceive.

"This is an especially important issue for women of color, and even more important for those who live in Northern climates or who wear head covers and do not get enough sun exposure, she told MedPage Today. "An even more important issue for African-American women because there is more lactose intolerance in this population, and if you are a young, African-American woman who lives in Detroit and is lactose intolerant you are very likely to be low in vitamin D."

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